The present invention relates to oriented, heat shrinkable thermoplastic film.
For many years, the heat shrinkable films industry has endeavored to reduce film gauge while maintaining performance in response to initiatives associated with source reduction. Lower gauges also allow for increased footage on rolls, which benefits the customer by reducing downtime (changeover time).
However, prior strategies involving films having single resin layers or conventional melt blends of resins, especially linear low density polyethylene (LLDPE), typically resulted in performance concessions. For instance, some improvement in optical quality and percent free shrink may have been seen, but with an undesirable degradation in impact strength. In the case of other blend compositions, good impact resistance and abrasion resistance could be obtained, but with an accompanying degradation in free shrink and clarity.
Thus, the technical challenge remained to design display films with higher impact resistance than LLDPE, but with optical and shrink properties comparable to LLDPE. The inventors have found that the use of a multicomponent ethylene/alpha-olefin interpenetrating network resin (herein xe2x80x9cIPN resinxe2x80x9d) permits property tailoring without compromising clarity, impact resistance, free shrink, or resistance to tear propagation. The result is a stronger, more abuse resistant film having the shrink and optical properties presently provided by LLDPE. These inventive films result in lower failures at a processor""s packaging machine, or during distribution of contents. The tensile strength of this film is superior to many conventional films, thereby permitting downgauging. A down-gauged film with performance properties comparable to prior LLDPE formulations at their previous gauge can account for less downtime and changeovers at the customer""s plant owing to the above mentioned greater roll footage. The ability to deliver higher value heat shrinkable film without a significant cast premium is a distinct advantage of the films of this invention. Other resins recognized for providing certain performance features (i.e. clarity, seal initiation temperature, low temperature shrink) such as metallocene resins, or blends, cannot provide abrasion resistance, nor deliver low temperature and low haze performance without a substantial cost penalty. The inventors have found that the use of an IPN resin in the skin layer or layers of a multilayer film can result in reduced levels of required antiblocking agent via removal of, or reduction in, the need for ethylene/vinyl acetate copolymer (EVA) in the skin layers. Additive reduction is advantageous because of reduced abrasive damage from inorganic particles, reduced accumulation of wax onto machine and package contents due to transfer from film to contact surfaces, together with better gloss and clarity.
The use of IPN resins thus provides superior mechanical strength properties while preserving excellent optical and shrink values comparable to e.g. D955(trademark) film. These performance attributes can lead to higher performance films at comparable thickness relative to current LLDPE films or to thinner films.
An improved packaging film in accordance with the present invention can thus provide adequate resistance to tear propagation; excellent free shrink; good optics, including haze, clarity, and gloss values; high impact resistance; and high tensile strength.
In a first aspect, a multilayer oriented heat shrinkable film comprises an internal layer comprising a multicomponent ethylene/alpha-olefin interpenetrating network resin having a melt index less than 1.4, the resin comprising a homogeneous component having a melt index of less than 1, and a density of at least 0.91 grams/cubic centimeter, and a heterogeneous component having a melt index of between 1.5 and 20; and outer layers comprising a polymeric resin; wherein the film has a haze value (ASTM D 1003-95) less than or equal to 5, a peak load/mil value (ASTM D 3763-95a) of at least 155 newtons/mil, and a free shrink (ASTM D 2732-83) at a temperature of 200xc2x0 F. of at least 8% in either or both of the longitudinal and transverse directions.
In a second aspect, a multilayer oriented heat shrinkable film comprises an internal layer comprising polymeric resin; first and second intermediate layers, each disposed on a respective opposite side of the internal layer, comprising multicomponent ethylene/alpha-olefin interpenetrating network resin having a melt index less than 1.4, the resin comprising a homogeneous component having a melt index of less than 1, and a density of at least 0.91 grams/cubic centimeter, and a heterogeneous component having a melt index of between 1.5 and 20; and first and second outer layers, disposed on the first and second intermediate layers respectively, comprising a polymeric resin; wherein the film has a haze value (ASTM D 1003-95) less than or equal to 5, a peak load/mil value (ASTM D 3763-95a) of at least 155 newtons/mil, and a free shrink (ASTM D 2732-83) at a temperature of 200xc2x0 F. of at least 8% in either or both of the longitudinal and transverse directions.
In a third aspect, a solid state oriented heat shrinkable film comprises between 50 and 100 percent, by volume of the total film, of a multicomponent ethylene/alpha-olefin interpenetrating network resin having a melt index less than 1.4, the resin comprising a homogeneous component having a melt index of less than 1, and a density of at least 0.91 grams/cubic centimeter, and a heterogeneous component having a melt index of between 1.5 and 20; and between 0 and 50 percent, by volume of the total film, of a polymeric resin; wherein the film has a haze value (ASTM D 1003-95) less than or equal to 5, a peak load/mil value (ASTM D 3763-95a) of at least 155 newtons/mil, and a free shrink (ASTM D 2732-83) at a temperature of 200xc2x0 F. of at least 8% in either or both of the longitudinal and transverse directions.
In a fourth aspect, a multilayer oriented heat shrinkable film comprises an internal layer comprising a blend of a multicomponent ethylene/alpha-olefin interpenetrating network resin having a melt index less than 1.4, and ethylene polymer or copolymer having a density of at least 0.935 grams/cubic centimeter, wherein the multicomponent ethylene/alpha-olefin interpenetrating network resin having a melt index less than 1.4 comprises a homogeneous component having a melt index of less than 1, and a density of at least 0.91 grams/cubic centimeter, and a heterogeneous component with a melt index of between about 1.0 and 20; and outer layers comprising a polymeric resin; wherein the film has a haze value (ASTM D 1003-95) less than or equal to 5, a peak load/mil value (ASTM D 3763-95a) of at least 155 newtons/mil, and a free shrink (ASTM D 2732-83) at a temperature of 200xc2x0 F. of at least 8% in either or both of the longitudinal and transverse directions.
Definitions
xe2x80x9cAcrylicxe2x80x9d herein refers to acrylic or to methacrylic.
xe2x80x9cComposite free shrinkxe2x80x9d herein refers to a value determined by summing the percent free shrink in the longitudinal direction with the percentage free shrink in the transverse direction. For example, Example 1 in Table 3 displays the following values at 200xc2x0 F.: 17% free shrink in the longitudinal direction, and 19% free shrink in the transverse direction. The composite free shrink would then be 17% +19%, or a value of 36%.
xe2x80x9cCRYSTAFxe2x80x9d herein refers to an analytical technique which can be used to characterize the composition of a polymer by means of a fractionation scheme based on crystallization isolation. Samples were analyzed by Polymer Char (Valencia Parc Tecnologic, PO Box 176 E-46980, Paterna, Spain). The technique generates results equivalent to that provided from TREF. (see Monrabal (1994) J. Applied Poly. Sci. 52, 491; Soares et al., SPE Polyolefins XI p287-312).
xe2x80x9cEthylene/alpha-olefin copolymerxe2x80x9d (EAO) herein refers to copolymers of ethylene with one or more comonomers selected from C3 to C10 alpha-olefins such as propene, butene-1,hexene-1, octene-1, etc. in which the molecules of the copolymers comprise long polymer chains with relatively few side chain branches arising from the alpha-olefin which was reacted with ethylene. This molecular structure is to be contrasted with conventional high pressure low or medium density polyethylenes which are highly branched with respect to EAOs and which high pressure polyethylenes contain both long chain and short chain branches. EAO includes such heterogeneous materials as linear medium density polyethylene (LMDPE), linear low density polyethylene (LLDPE), and very low and ultra low density polyethylene (VLDPE and ULDPE), such as DOWLEX(trademark) or ATTANE(trademark) resins supplied by Dow, ESCORENE(trademark) or EXCEED(trademark) resins supplied by Exxon; as well as linear homogeneous ethylene/alpha olefin copolymers (HEAO) such as TAFMER(trademark) resins supplied by Mitsui Petrochemical Corporation, EXACT(trademark) resins supplied by Exxon, or long chain branched (HEAO) AFFINITY(trademark) resins supplied by the Dow Chemical Company, or ENGAGE(trademark) resins supplied by DuPont Dow Elastomers.
xe2x80x9cFree shrink balancexe2x80x9d herein refers to the value, which defines the percent of difference between the free shrink of a film in the longitudinal direction and the free shrink of the same film in the transverse direction at 240xc2x0 F., defined by the mathematical relationship:   [            "LeftBracketingBar"                        FS          TD                -                  FS          LD                    "RightBracketingBar"              FS      TD        ]
where:
FS=free shrink
TD=transverse direction
LD=longitudinal direction
Films of the present invention preferably exhibit a free shrink balance of less than or equal to 30%. As an example (see Table 3) the free shrink of Example 1 in the longitudinal direction is 43% at 240xc2x0 F. The free shrink of Example 1 in the transverse direction is 48% at 240xc2x0 F. Inserting these values in the above relationship:   [            "LeftBracketingBar"              48        -        43            "RightBracketingBar"        48    ]
i.e., the absolute value of 48 minus 43 equals 5, and 5 divided by 48 equals 104, which is equivalent to 10.4%, which is less than 30%.
xe2x80x9cHeat shrinkablexe2x80x9d herein refers to a property of a material which, when heated to a temperature of 200xc2x0 F., will exhibit a free shrink (ASTM D 2732-83) of at least 8% in the longitudinal direction, and/or at least 8% in the transverse direction. Heat shrinkable films of this invention are solid state oriented as contrasted to hot blown films which are melt state oriented.
xe2x80x9cHigh density polyethylenexe2x80x9d (HDPE) herein refers to a polyethylene having a density of between 0.94 and 0.965 grams per cubic centimeter.
xe2x80x9cIntermediatexe2x80x9d herein refers to a layer of a multi-layer film which is between an outer layer and an internal layer of the film.
xe2x80x9cInternal layerxe2x80x9d herein refers to a layer which is not an outer or surface layer, and is typically a central or core layer of a film.
xe2x80x9cLDxe2x80x9d herein refers to the longitudinal direction, i.e. the direction of the film parallel to the path of extrusion. xe2x80x9cTDxe2x80x9d herein refers to the transverse direction, i.e. the direction of the film transverse to the path of extrusion.
xe2x80x9cLinear low density polyethylenexe2x80x9d (LLDPE) herein refers to polyethylene having a density between 0.917 and 0.925 grams per cubic centimeter.
xe2x80x9cLinear medium density polyethylenexe2x80x9d (LMDPE) herein refers to polyethylene having a density between 0.926 grams per cubic centimeter and 0.939 grams per cubic centimeter.
xe2x80x9cMulticomponent ethylene/alpha-olefin interpenetrating network resinxe2x80x9d or xe2x80x9cIPN resinxe2x80x9d herein refers to multicomponent molecular mixtures of polymer chains. Because of molecular mixing, IPN resins cannot be separated without breaking chemical bonds. Polymer chains combined as IPN resins are interlaced at a molecular level and are thus considered true solid state solutions. Interpenetrating networks, unlike blends, become new compositions exhibiting properties distinct from parent constituents. Interpenetrating networks provide phase co-continuity leading to surprising enhancement of physical properties. Due to the mixture of at least two molecular types, these compositions may exhibit bimodal or multimodal curves when analyzed using TREF or CRYSTAF. Interpenetrating networks as herein used includes semi-interpenetrating networks and therefore describes crosslinked and uncrosslinked multicomponent molecular mixtures having a low density fraction and a high density fraction. Specific production methods for preparing IPN resins useful for carrying out the present invention, are disclosed in U.S. Pat. No. 5,747,594 (deGroot et al.), U.S. Pat. No. 5,370,940 (Hazlitt et al.), and WO 94/17112 (Kolthammer), all herein incorporated by reference in their entirety. Preferred IPN resins are prepared using a parallel or sequential multiple reactor scheme. Especially preferred IPN resins are produced from a solution polymerization scheme. Alternatively, IPNs useful for the inventive films may be prepared within a single reactor by completing polymerization of the heterogeneous component prior to initiating the polymerization of the homogeneous component. Examples of catalysts suitable for preparing the heterogeneous component are described in U.S. Pat. Nos. 4,314,912 (Lowery et al.), U.S. Pat. No. 4,547,475 (Glass et al.), and U.S. Pat. No. 4,612,300 (Coleman, III); examples of catalysts suitable for producing the homogeneous component are described in U.S. Pat. Nos. 5,026,798 and 5,055,438 (Canich); 3,645,992(Elston); 5,017,714 (Welborn); and 4,076,698 (Anderson); all herein incorporated by reference in their entirety.
xe2x80x9cOuter layerxe2x80x9d herein refers to what is typically an outermost, usually surface layer or skin layer of a multi-layer film, although additional layers, coatings, and/or films can be adhered to it.
xe2x80x9cPolymerxe2x80x9d herein refers to homopolymer, copolymer, terpolymer, etc. xe2x80x9cCopolymerxe2x80x9d herein includes copolymer, terpolymer, etc.
xe2x80x9cSolid-state orientationxe2x80x9d herein refers to the orientation process carried out at a temperature higher than the highest Tg (glass transition temperature) of resins making up the majority of the structure and lower than the highest melting point, of at least some of the film resins, i.e. at a temperature at which at least some of the resins making up the structure are not in the molten state. Solid state orientation may be contrasted to xe2x80x9cmelt state orientationxe2x80x9d i.e. including hot blown films, in which stretching takes place immediately upon emergence of the molten polymer film from the extrusion die.
xe2x80x9cSolid state orientedxe2x80x9d herein refers to films obtained by either coextrusion or extrusion coating of the resins of the different layers to obtain a primary thick sheet or tube (primary tape) that is quickly cooled to a solid state to stop or slow crystallization of the polymers, thereby providing a solid primary film sheet, and then reheating the solid primary film sheet to the so-called orientation temperature, and thereafter biaxially stretching the reheated film sheet at the orientation temperature using either a tubular solid-state orientation process (for example a trapped bubble method) or using a simultaneous or sequential tenter frame process, and finally rapidly cooling the stretched film to provide a heat shrinkable film. In the trapped bubble solid state orientation process the primary tape is stretched in the transverse direction (TD) by inflation with air pressure to produce a bubble, as well as in the longitudinal direction (LD) by the differential speed between the two sets of nip rolls that contain the bubble. In the tenter frame process the sheet or primary tape is stretched in the longitudinal direction by accelerating the sheet forward, while simultaneously or sequentially stretching in the transverse direction by guiding the heat softened sheet through a diverging geometry frame.
xe2x80x9cSubstantially balanced free shrinkxe2x80x9d herein refers to film of the invention characterized by a free shrink balance less than or equal to 30%. As an example, Example 1 was calculated above as having a free shrink balance of 10.4% at 240xc2x0 F. Since this value is less than or equal to 30%, Example 1 has a substantially balanced free shrink at 240xc2x0 F.
All compositional percentages used herein are presented on a xe2x80x9cby weightxe2x80x9d basis, unless designated otherwise.